1. Field of the Invention
This invention relates to phosphate ester base stock compositions comprising mixed n-butyl/isobutyl phosphate esters and to aircraft hydraulic fluid compositions comprising such base stocks.
2. State of the Art
Hydraulic fluids used in the hydraulic systems of aircraft must meet exacting specifications set by aircraft manufacturers. Accordingly, the components of aircraft hydraulic fluids are carefully chosen to balance, among other properties, stability, compatibility, density, toxicity and the like. Whether the selected components can, in fact, be balanced to meet these specifications is unpredictable. Moreover, the amounts of individual components used in compositions which meet the specifications is not a priori predictable.
Trialkyl phosphate esters, such as tri-n-butyl phosphate and triisobutyl phosphate, have been used previously as base stocks for aviation hydraulic fluids. For example, trialkyl phosphate ester base stocks are described in U.S. Pat. No. 5,464,551, the disclosure of which is incorporated herein by reference in its entirety. In particular, low density aviation hydraulic fluids, i.e., fluids having a density below about 1.020 g/L at 25xc2x0 C., have conventionally been prepared using tri-n-butyl phosphate as the major component of the base stock. However, tri-n-butyl phosphate is known to be a skin irritant and minimizing its concentration is desirable. Alternatively, low density fluids employing triisobutyl phosphate as the major component have had difficulty meeting the low volatility and low temperature viscosity requirements imposed on aviation hydraulic fluids.
It has now been discovered that phosphate ester base stocks comprising mixed isobutyl/n-butyl phosphate esters, i.e., n-butyl diisobutyl phosphate or di-n-butyl isobutyl phosphate or mixtures thereof, have surprising and unexpected properties when compared to base stocks containing major amounts of tri-n-butyl phosphate and triisobutyl phosphate or physical mixtures thereof. Specifically, it has been found that by employing mixed isobutyl/n-butyl phosphate esters in the base stock of the fluid, an unexpected, surprising balance of properties critical to aviation hydraulic fluids is obtained, including acceptable hydrolytic stability, high flash point, good anti-wear properties, acceptable erosion protection, acceptable low temperature flow properties (viscosity), and elastomer compatibility.
This invention is directed to phosphate ester base stock compositions comprising n-butyl diisobutyl phosphate or di-n-butyl isobutyl phosphate or a mixture thereof, and to aircraft hydraulic fluid compositions containing such base stock compositions.
Accordingly, in one of its composition aspects, the present invention is directed to an aircraft hydraulic fluid composition comprising:
(a) from about 30 to about 95 weight percent, based on the total weight of the fluid, of a phosphate ester selected from the group consisting of n-butyl diisobutyl phosphate, di-n-butyl isobutyl phosphate and mixtures thereof;
(b) from 0 to about 15 weight percent, based on the total weight of the fluid, of one or more triaryl phosphates;
(c) an effective amount of a viscosity index improver;
(d) an effective amount of acid control additive; and
(e) an effective amount of an erosion inhibitor.
Preferably, the aircraft hydraulic fluid comprises from about 30 to about 90 weight percent of a phosphate ester selected from the group consisting of n-butyl diisobutyl phosphate, di-n-butyl isobutyl phosphate and mixtures thereof, based on the total weight of the fluid.
In a preferred embodiment, the aircraft hydraulic fluids of this invention further comprise:
(f) an effective amount of a rust inhibitor or a mixture of rust inhibitors; and
(g) an effective amount of an antioxidant or a mixture of antioxidants.
In a preferred embodiment, the present invention is directed to an aircraft hydraulic fluid composition comprising about 30 to about 95 weight percent, based on the total weight of the fluid, of a phosphate ester base stock comprising a phosphate ester selected from the group consisting of n-butyl diisobutyl phosphate, di-n-butyl isobutyl phosphate and mixtures thereof, and a sufficient amount of one or more triaryl phosphates such that the base stock composition produces no more than 25% elastomer seal swell; an effective amount of a viscosity index improver; an effective amount of acid control additive; and an effective amount of an erosion inhibitor.
In another preferred embodiment, the present invention is directed to an aircraft hydraulic fluid composition comprising about 30 to about 95 weight percent, based on the total weight of the fluid, of a phosphate ester base stock comprising from about 4 to about 14 weight percent, based on the total weight of the fluid, of one or more triaryl phosphates, the remainder of the base stock comprising a phosphate ester selected from the group consisting of n-butyl diisobutyl phosphate, di-n-butyl isobutyl phosphate and mixtures thereof; an effective amount of a viscosity index improver; an effective amount of acid control additive; and an effective amount of an erosion inhibitor.
In yet another preferred embodiment, the present invention is directed to an aircraft hydraulic fluid comprising:
(a) from about 30 to about 95 weight percent, based on the total weight of the fluid, of a phosphate ester selected from the group consisting of n-butyl diisobutyl phosphate, di-n-butyl isobutyl phosphate and mixtures thereof;
(b) from about 0 to about 15 weight percent, based on the total weight of the fluid, of one or more triaryl phosphates;
(c) from about 4 to 6 weight percent, based on the total weight of the fluid, of a viscosity index improver;
(d) from about 5 to 6.5 weight percent, based on the total weight of the fluid, of an acid control additive;
(e) from about 0.05 to about 0.1 weight percent, based on the total weight of the fluid, of an erosion inhibitor;
(f) from about 0.005 to about 0.5 weight percent, based on the total weight of the fluid, of a rust inhibitor or a mixture of rust inhibitors; and
(g) from about 0.5 to about 2.5 weight percent, based on the total weight of the fluid, of an antioxidant or a mixture of antioxidants.
In one embodiment of the present invention, the aircraft hydraulic fluid further comprises from about 1 to about 30 weight percent of triisobutyl phosphate based on the total weight of the fluid.
In another embodiment, the aircraft hydraulic fluid comprises less than 15 weight percent, preferably less than 5 weight percent, of tri-n-butyl phosphate based on the total weight of the fluid.
In another of its composition aspects, this invention is directed to a phosphate ester base stock for use in aircraft hydraulic fluids comprising:
(a) from about 50 to 100 weight percent, based on the total weight of the base stock, of a phosphate ester selected from the group consisting of n-butyl diisobutyl phosphate, di-n-butyl isobutyl phosphate and mixtures thereof;
(b) from 0 to about 15 weight percent, based on the total weight of the base stock, of one or more triaryl phosphates.
Preferably, the phosphate ester base stock comprises from 60 to 100 weight percent, more preferably from 80 to 100 weight percent, and still more preferably from 85 to 100 weight percent, based on the total weight of the base stock, of a phosphate ester selected from the group consisting of n-butyl diisobutyl phosphate, di-n-butyl isobutyl phosphate and mixtures thereof;
In a preferred embodiment, this invention is directed to a phosphate ester base stock for use in aircraft hydraulic fluids comprising a phosphate ester selected from the group consisting of n-butyl diisobutyl phosphate, di-n-butyl isobutyl phosphate and mixtures thereof, and a sufficient amount of one or more triaryl phosphates such that the base stock composition produces no more than 25% elastomer seal swell.
In another of its composition aspects, this invention is directed to a phosphate ester base stock for use in aircraft hydraulic fluids comprising from about 5 to about 15 weight percent, based on the total weight of the base stock, of one or more triaryl phosphates, the remainder of the base stock comprising a phosphate ester selected from the group consisting of n-butyl diisobutyl phosphate, di-n-butyl isobutyl phosphate and mixtures thereof.